Many new heat transfer apparatuses and techniques have been developed over recent years in order to meet the growing needs for heat dissipation in more complex and denser electrical systems, electronic components and circuits thereof. Such techniques can be generally classified as either direct or indirect heat transfer.
Direct cooling techniques involve the use of a coolant medium, e.g. air or liquid, applied directly to the component or circuit for direct cooling. The disadvantage, of course, is that the component or circuit must be able to withstand such direct contact without adversely affecting the component or circuit. Moreover, in the case of using a liquid coolant, additional means is often required for collecting and possibly recycling the coolant medium.
Indirect cooling techniques usually involve a heat transfer fluid that is provided within a heat transfer device such that the heat transfer fluid dissipates or supplies heat through a structural component, such as a wall, of the heat transfer device. Typically, the heat transfer device is positioned so that the structural component or wall comes into direct contact with the component to be thermally affected. Such indirect heat transfer devices are defined as either: 1) a closed-type device, wherein a quantity of a heat transfer medium is sealed within the closed structure of the heat transfer device, and heat is transferred through the device between the component to be cooled and a heat sink which is otherwise cooled; or 2) a fluid circulating heat transfer type device, wherein the heat transfer fluid flows along a path defined within the device as part of a circuit when it is positioned against a component to be cooled. Fluid circulating heat transfer devices are typically provided as a component within a fluid circuit connected between a fluid source and drain.
A known thermal transfer device of the closed variety has recently been developed by the assignee of the present invention, Minnesota Mining and Manufacturing Company, and is described in U.S. Pat. No. 4,997,032 granted Mar. 5, 1991 to Danielson et al. The thermal transfer device is a thermal transfer bag, also known as the Liquid Heat Sink bag available from Minnesota Mining and Manufacturing Company as models Fluorinert.TM. Liquid Heat Sink FC-3260 and FC-3261. The bag is made of a sheet of flexible, durable, air-impermeable material and is filled with a thermally conductive, chemically inert, essentially gas free body of liquid which comprises fluorochemical liquid. The bag is placed between the heat generating component and a heat dissipating body, and conduction through the liquid as well as some movement of liquid within the bag due to convection currents transfer heat from the component to the heat dissipating body.
The aforementioned thermal transfer bag is advantageous in that the flexible material geometrically conforms to the configuration of the cavity within the electronic equipment, and comes into intimate contact with the heat generating components and the heat dissipating body to establish a thermal path therebetween. Moreover the inherent shock absorbing nature of a filled bag functions as a packing or cushion to help it protect the component from physical shock damage. The bag may be removed and replaced in the field during repair.
A modification of the above-described thermal transfer bag is described in U.S. Pat. No. 5,000,256, granted Mar. 19, 1991 to Tousignant and also owned by the assignee of the present invention. The modified thermal transfer bag is further provided with at least one metallic thermal via for improved heat transfer. The metallic thermal via extends through a hole in the bag for direct contact with an external surface of a heat generating component, and a portion of the via that extends within the bag functions as a heat radiating fin to enhance the transfer of heat to the liquid within the bag.
The thermal transfer bags disclosed in U.S. Pat. Nos. 4,997,032 and 5,000,256, however, can be difficult to use in certain situations where the thermal transfer bag is to be inserted between components that are relatively spatially fixed in place. As there becomes a need for increased quantities of heat transfer fluid within the bag for any reason, or in order to fill a wider gap between components, the increased quantity of fluid makes the thermal transfer bags rounder and eventually circular in transverse cross section. In other words, as the bags are filled with more heat transfer fluid they react similar to a balloon. Thus, as greater quantities are desired, it becomes increasingly difficult to fit such thermal transfer bags in the spaces between components and the effectiveness thereof is reduced along with their surface contact.
Other heat sink pillows or bags suffering from at least the aforementioned deficiency of thermal transfer bags are described in U.S. Pat. No. 3,586,102, granted Jun. 22, 1971 to Gilles and IBM Technical Disclosure Bulletin Vol. 19 No. 8, January, 1977. In each of these cases, a pillow or sack is filled with thermally conductive grease or liquids. Moreover, the grease or liquid is contained by one or more film layers.
Another type of heat transfer device is shown and described in U.S. Pat. No. 4,092,697, granted May 7, 1978 to Spaight. Specifically, a formable film layer is mounted on the underside of a rigid cover, wherein when the rigid cover is attached over a substrate with circuit devices thereon, the film containing a thermally conductive material between it and the rigid cover contacts the top surface of the circuit device. The thermally conductive material between the film and the rigid cover transfers heat generated from the device through the film layer to the rigid cover which is further provided with cooling fins. The rigid cover acts as a heat sink. This type of heat transfer device is even more limited in applications than the thermal transfer bags discussed above in that the thermal conductive material is provided within a film sack attached to a rigid cover assembly which must be mounted to the substrate to which circuit devices are mounted. There is very little versatility to such devices.
A similar heat transfer structure is described in U.S. Pat. No. 4,323,914, granted Apr. 6, 1982 to Berndlmaier. In this case, either the thermally conductive material is maintained within a film like sack attached to a rigid cover which acts as a heat sink and is secured to the circuit substrate, or the circuit device is coated with a protective layer so that thermal transfer liquid can contact the protective layer without detrimentally affecting or contaminating the circuit device.
Disclosed in U.S. Pat. No. 4,155,402, granted May 22, 1979 to Just is a cooling device including a compliant surface which contacts electric circuit components for cooling. A rigid channeled cooling plate is provided with a recessed surface, and the recessed surface is filled with a paste-like heat conductive material which is covered by a thin film attached to the base by adhesive. The film layer and the heat conductive paste within the rigid recessed cooling plate comprise the compliant surface. The Just device, however, is disadvantageous in that a very rigid device is created with only a compliant single surface. Moreover, such heat transfer devices are limited in applicability due to the rigid structural requirements thereof.
In general, the aforementioned prior art heat transfer devices lack the versatility to be applicable over a wide range of spaces between components that are to be thermally affected, i.e. heated or cooled, and which can be easily manufactured and replaced as necessary. Furthermore, very flexible heat transfer devices, such as the thermal transfer bags described above, cannot be easily used in relatively narrow or in relatively wide open spaces between components, while the very rigid type heat transfer devices are not versatile enough for such application.